To have a consistent interface, so that there is no different interface that you need to implement for Stateful Session Bean and Stateless Session Bean. Both Stateless and Stateful Session Bean implement javax.ejb.SessionBean and this would not be possible if stateless session bean is to remove ejbActivate and ejbPassivate from the interface. You could argue that the two (stateful and stateless) are so different that they should have their own interface but Sun did not think so. They made both session beans implement the same interface and provided deployment descriptor to denote which one is it that you are deploying.
